Most of the compressors used for a refrigerant cycle have a compression section and a motor chamber in a closed vessel, and the compression section is driven via the rotary drive shaft of a motor. The compression section introduces a low-pressure refrigerant into a closed working chamber formed by engaging scroll wraps erected on the end plates of a fixed scroll and an orbiting scroll with each other, and compresses the refrigerant to a high-pressure refrigerant by decreasing the closed volume of the closed working chamber.
When the refrigerant is compressed in the closed working chamber, the pressure in the working chamber is increased, so that a force that separates the orbiting scroll from the fixed scroll acts. When the orbiting scroll is pushed back downward, a minute gap is formed between the orbiting scroll and the fixed scroll, and therefore the refrigerant leaks out through this gap, which may decrease the compression efficiency.
Accordingly, there has been proposed a configuration in which to prevent the orbiting scroll from separating, some of the discharge pressure of the closed working chamber is led to the back surface of the end plate of the orbiting scroll and is utilized as a back pressure. According to this configuration, the orbiting scroll can be pressed against the fixed scroll side.
However, in the case where the orbiting scroll is pressed by utilizing only the high discharge pressure, under an operating condition that the discharge pressure is higher than the rated pressure, the pressing force of orbiting scroll is too high, and hence the performance and the reliability decrease. Inversely, under an operating condition that the discharge pressure is lower than the rated pressure, the pressing force is too low, so that the orbiting scroll is liable to separate.
To solve this problem, for example, as described in Patent Document 1 (Japanese Patent Application Publication No. 2000-161254), a method has been proposed in which a central portion of the back surface of the end plate of the orbiting scroll is pressed by the discharge pressure, and further the outer periphery thereof is pressed by a medium pressure between the discharge pressure and the suction pressure. According to this method, even if the discharge pressure is higher or lower than the rated pressure, an excessive increase or decrease in pressing force can be eased.
However, the conventional scroll compressor has the problems as described below. In the configuration described in Patent Document 1, the inside of the small-diameter portion of a thrust ring is also used as a main guide bearing, or an Oldham's ring is supported by utilizing the upper end face of the large-diameter portion of the thrust ring, so that the construction of the thrust ring itself is complicated, which presents a problem of high manufacturing cost.
Also, since the thrust ring is configured so that a balancer is arranged on the inside, the diameter of a medium-diameter seal portion of the thrust ring increases inevitably. Further, since the large-diameter portion of thrust ring provided further on the outside of the medium-diameter seal portion is utilized as a means for regulating the moving distance in the axial direction of the thrust ring, the diameter of the thrust ring itself is large, so that the size and weight of the compressor increase inevitably.